1. Field of the Invention
The present invention relates to a laser scanning unit and an electrophotographic image forming apparatus adopting the same, and more particularly, to a laser scanning unit having a plurality of rotatable polygonal mirrors actuated by one driving unit, and an electrophotographic image forming apparatus adopting the same.
2. Description of the Related Art
FIG. 1 is a cross-sectional view of a conventional color electrophotographic image forming apparatus. As shown in FIG. 1, a charger 15, a plurality of laser scanning units 30, a plurality of developing units 17, a transfer roller 21, and a fusing roller 22 are disposed around a photoreceptor web 14, which is supported by three rollers 11, 12, and 13 to circulate along an endless (closed) path.
In the above structure, if light emitted from the laser scanning units 30 is scanned onto the photoreceptor web 14, which is uniformly charged to a predetermined electrostatic potential by the charger 15, an electrostatic latent image corresponding to image information is formed by a relative electrostatic potential difference between a first portion where the light emitted from the laser scanning units 30 is scanned, and a second portion where the light emitted from the laser scanning units 30 is not scanned. Then, toner is supplied to the electrostatic latent image from the developing units 17 to form a toner image. A print of the toner image formed on the photoreceptor web 14 is completed by transferring the toner image to the transfer roller 21 rotating while being in contact with the photoreceptor web 14, and then transferring the toner image from the transfer roller 21 to a sheet of print paper S inserted between the transfer roller 21 and the fusing roller 22 pressing the print paper S against the transfer roller 21 so that the toner image is fixed to the printer paper S by heat and pressure.
FIG. 2 is a perspective view showing an example of a laser scanning unit used in the conventional color electrophotographic image forming apparatus presented in FIG. 1. As shown in FIGS. 1 and 2, each of the laser scanning units 30 includes a photogenerator 31, a rotatable polygonal mirror 32, a lens unit 33, and a motor 36. The rotatable polygonal mirror 32 is provided with a plurality of reflecting surfaces and coupled with a rotation shaft of the motor 36 rotating the rotatable polygonal mirror 32. The light, which is emitted from the photogenerator 31 and is incident onto the reflecting surfaces of the rotatable polygonal mirror 32, is deflected in a main scanning direction M due to a rotation of the rotatable polygonal mirror 32, and the deflected light is incident onto the photoreceptor web 14 via the lens unit 33.
In order to form a color image, the four laser scanning units 30 are required to respectively scan electrostatic latent images corresponding to four colors (yellow, magenta, cyan, and black) onto the photoreceptor web 14. Further, a synchronous detecting unit 34 matching synchronization between the light scanned onto the photoreceptor web 14 in the main scanning direction M and a motor driver 37 controlling a rotation of the motor 36 is required to control individual scanning operations of the respective laser scanning units 30. The synchronous detecting unit 34 and the motor driver 37 are controlled together by a system control unit 35.
In a case where the image forming apparatus has the four laser scanning units 30 as described above, a size of the image forming apparatus must become large. Thus, such a large image forming apparatus is difficult to satisfy a recent trend toward a small and light apparatus. Further, since each of the laser scanning units 30 has the lens unit 33 and the motor driver 37, costs of components for the laser scanning units 30 and an assembly cost thereof also increase. Further, a color registration error may be generated by control errors of the respective motor 36.
In order to solve the above-described problems, as shown in FIG. 3, there is proposed to install a plurality of reflecting mirrors 43 in the four laser scanning units 30 such that a plurality of photogenerators 42 emit lights onto one rotatable polygonal mirror 41 in different directions and the emitted lights are scanned onto photoreceptor media 44 through different light paths, respectively.
However, since the reflecting mirrors 43 must be used in the above-proposed method, the reflecting mirrors 43 must be disposed not to block the light paths of the lights emitted from the photogenerators 42. Thus, there is a limitation to install the reflecting mirrors 43 in a narrow space of the four laser scanning units 30 while satisfying the above condition. As a result, the size of the image forming apparatus must become large. Further, in a case where an angular error occurs in the light paths when the reflecting mirrors 43 are installed, the light path deviates twice as much as the angular error of an installation angle of the reflecting mirrors 43, thereby causing the color registration error of the color image. Accordingly, a great amount of time is required for accurately assembling and adjusting the reflecting mirrors 43, thereby increasing the assembly cost thereof.